1. Field of the Invention
The invention relates to a structure for joining an impeller rotatable at a high speed to a rotatable shaft.
2. Description of the Related Art
A structure for joining an impeller rotatable at a high speed to a rotatable shaft to be used, for instance, in a supercharger or a fan is required to have such basic characteristics as an ability for transferring a torque between an impeller and a shaft, a self-align ability for self-aligning an impeller with a shaft, and serviceability. Herein, the ability for transferring a torque between an impeller and a shaft includes an ability for transferring a torque in usual conditions such as steady rotation, acceleration and deceleration, and an ability of avoiding instantaneous elimination of torque transfer ability for preventing over-speeding in unusual conditions such as when an impeller is damaged or when foreign material enters the impeller structure. The self-aligning ability includes an ability that an impeller and a shaft are self-aligned with each other when assembled in a stationary condition to thereby easily keep rotation balance, and an ability of absorbing therein deformation of an impeller and/or a shaft caused by a centrifugal force produced by rotation. Herein, the deformation caused by a centrifugal force includes, for instance, an expansion in the inner diameter and a shrinkage in the axial length of the impeller. The above mentioned serviceability includes an ability of easily assembling and disassembling.
Various attempts have been made to satisfy the above mentioned requirements. Examples of conventional structures for joining an impeller to a shaft are illustrated in FIGS. 1A, 1B and 1C. FIG. 1A illustrates a conventional structure in which an impeller 1 is joined to a shaft 2 by means of shrink fitting. FIG. 1B illustrates another conventional structure in which an impeller 1 is joined to a shaft by means of a combination of a radial key 4, a spline 5, and a nut 6. FIG. 1C illustrates still another conventional structure in which an impeller is joined to a shaft 2 by means of a radial key 4, a tapered bushing 7, and a nut 6.
The structure illustrated in FIG. 1A has problems as follows. Since a torque is transferred through shrink fitting, a capacity of the torque is not so large as the other structures in usual. In addition, if an excessive torque is generated due to foreign material entering the impeller, the shrink fitting is forced into a slippage condition, and hence an ability of transferring a torque is instantaneously eliminated. Furthermore, the shrink fitting makes it quite difficult to assemble the impeller 1 to the shaft 2 and to disassemble the impeller 1 out of the shaft 2.
The structure illustrated in FIG. 1B has a problem of low self-aligning ability in the spline 5. That is, it is necessary that a gap exists between the top and bottom of the spline 5, in order to mesh teeth of the spline 5 with each other to thereby transfer a torque. This reduces self-aligning accuracy in the spline 5. If the spline 5 was formed with a fit section only for self-alignment, there would arise a problem of reduction in the easiness of assembling the spline 5.
In the structure illustrated in FIG. 1C, the tapered bushing 7 disposed between the impeller 1 and the rotatable shaft 2 increases the self-aligning ability of the structure. Hence, it is difficult to accurately position the impeller 1 axially. In order to enhance the positioning accuracy of the impeller 1, parts constituting the structure are required to be fabricated with high accuracy, which would bring a problem of increased fabrication costs.